Have a look at following bulletin. In CISCO-SLB-MIB, there's a ciscoSlbRealStateChange trap.
http://www.cisco.com/en/US/products/hw/modules/ps2706/prod_bulletin09186a0080107b2b.html
ciscoSlbRealStateChange NOTIFICATION-TYPE
OBJECTS {
slbRealState
}
STATUS current
DESCRIPTION
"The notification generated when a real server
changes to a new state. The value of
slbRealServerState indicates the new state."
::= { ciscoSlbMIBNotifications 2 }
Cat6509(config)#snmp-server enable traps slb ?
csrp Enable SNMP SLB Fault Tolerance Replication Protocol traps
real Enable SNMP SLB Real Server traps
virtual Enable SNMP SLB Virtual Server traps